Apple's iPad is still leading the pack when it comes to tablet market share, but this is slowly changing as Samsung plays catch-up.

According to the International Data Corporation (IDC), a total of 52.5 million tablets were shipped in Q4 2012. Of this, 22.9 million were Apple's iPad, which represented a 48.1 percent growth from Q4 2011.

However, Apple's iPad market share has been slipping slightly on a quarterly basis. It went from 46.4 percent in Q3 2012 to 43.6 percent in Q4 2012.

Part of the reason for the slip is Samsung's ability to gain some ground with its Android and Windows 8-powered tablets. Samsung represented the No. 2 spot with a 263 percent year-on-year growth. For Q4 2012, it shipped 8 million tablets and snagged 15.1 percent of the market share.

Earlier this month, it was reported that the iPad's market share in Japan was bested by the Android-powered Nexus 7 tablet.

The top five tablet vendors (from No. 1 to No. 5) include Apple, Samsung, Amazon, Barnes & Noble and a collected group of "others."

Microsoft is now in on the tablet race too, with its first entry -- Microsoft Surface. In November 2012, it released Surface with Windows RT for ARM-based tablets. On February 9, it will release Surface with Windows 8 Pro.

While Microsoft's Surface with Windows RT was released in the fourth quarter, it was just shy of the top five list with IDC's calculated number of 900,000 shipments.

According to market research firm iSuppli, Microsoft Surface RT shipments into the channel for the fourth quarter were about 1.25 million, but sales out of the channel were only about 55-60 percent of that. This equals about 680,000-750,000 unit shipments, which is well below the 1 million mark.

The Surface with Windows RT has a lot stacked against it, such as a high return rate (iSuppli said that's mainly due to the steep learning curve of Windows 8), the fact that device makers aren't particularly interested in the OS and Microsoft failed to sell the device outside of its own kiosks (there are only a little over 60 of them in the U.S.) and online until mid-December.